I'd go with the misunderstanding. There is some reasoning for sloping a cut for water runoff, but perpendicular to the fibers reduces overall surface area of the cut, so sometimes the tradeoff is not worth it.
The article you saw was probably Guy M's and I've seen it before long before he published it. The reason here is because you are cutting at a bid branch node where there is no collar to cut to.
Because of it is seldom needed and time limits on dialog, proper pruning has been condensed to collar cuts. This is problematic because in storm mitigation you end up butchering trees more then need. Nodal pruning is in the standards. (not implying anyone said it was not, just on the soapbox)
Then there is the more frequent case where there is no collar due to the branch originating from a bud secondary to the terminal.
I call this primary and secondary branching, primary is the above mentioned, where the branch pith is conjoined with the stem. They are effectively codominant, though in a branch:stem relationship. They usually have much more vigor and caliper then a secondary (collared) branch and there is no branch protection zone in the fibers.
IMO primary branches should be removed, or controlled young, but I've digressed enough and will leave this for future discussion.